/usr/lib/hugs/packages/mtl/Control/Monad/List.hs is in libhugs-mtl-bundled 98.200609.21-5.4build1.
This file is owned by root:root, with mode 0o644.
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-- Needed for the same reasons as in Reader, State etc
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.List
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology, 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (multi-parameter type classes)
--
-- The List monad.
--
-----------------------------------------------------------------------------
module Control.Monad.List (
ListT(..),
mapListT,
module Control.Monad,
module Control.Monad.Trans,
) where
import Prelude
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Cont
import Control.Monad.Error
-- ---------------------------------------------------------------------------
-- Our parameterizable list monad, with an inner monad
newtype ListT m a = ListT { runListT :: m [a] }
instance (Monad m) => Functor (ListT m) where
fmap f m = ListT $ do
a <- runListT m
return (map f a)
instance (Monad m) => Monad (ListT m) where
return a = ListT $ return [a]
m >>= k = ListT $ do
a <- runListT m
b <- mapM (runListT . k) a
return (concat b)
fail _ = ListT $ return []
instance (Monad m) => MonadPlus (ListT m) where
mzero = ListT $ return []
m `mplus` n = ListT $ do
a <- runListT m
b <- runListT n
return (a ++ b)
instance MonadTrans ListT where
lift m = ListT $ do
a <- m
return [a]
instance (MonadIO m) => MonadIO (ListT m) where
liftIO = lift . liftIO
instance (MonadReader s m) => MonadReader s (ListT m) where
ask = lift ask
local f m = ListT $ local f (runListT m)
instance (MonadState s m) => MonadState s (ListT m) where
get = lift get
put = lift . put
instance (MonadCont m) => MonadCont (ListT m) where
callCC f = ListT $
callCC $ \c ->
runListT (f (\a -> ListT $ c [a]))
instance (MonadError e m) => MonadError e (ListT m) where
throwError = lift . throwError
m `catchError` h = ListT $ runListT m
`catchError` \e -> runListT (h e)
mapListT :: (m [a] -> n [b]) -> ListT m a -> ListT n b
mapListT f m = ListT $ f (runListT m)
|